1. Field of the Invention
This invention relates to the field of gas purification, and to the purification of hydride gases that are useful in the microelectronics manufacturing industry. More specifically, the invention relates to methods and materials for selectively removing trace amounts of oxygen impurities and moisture from hydride gases.
2. Description of the Prior Art
The provision of high purity gas streams is critically important in a wide variety of industrial and research applications. The rapid expansion of vapor-phase processing techniques, e.g. chemical vapor deposition, in the semiconductor industry has been associated with the deployment and use of manufacturing equipment that is totally reliant on the delivery of ultra-high purity process gases at the point of use in the semiconductor manufacturing facility.
Considering the impurities which are present in gas streams involved in semiconductor manufacturing, it is to be noted that the growth of high quality thin film electronic and optoelectronic cells by chemical vapor deposition or other vapor-based techniques is inhibited by a variety of low-level process impurities. These impurities can cause defects that reduce yields by increasing the number of rejects, which can be very expensive. These impurities may be particulate or chemical contaminants.
Chemical impurities may originate in the production of the source gas itself, as well as in its subsequent packaging, shipment, storage, and handling. Although source gas manufacturers typically provide analyses of source gas materials delivered to the semiconductor manufacturing facility, the purity of the gases may change because of leakage into or outgassing of the containers, e.g. gas cylinders, in which the gases are packaged. Impurity contamination may also result from improper gas cylinder changes, leaks into downstream processing equipment, or outgassing of such downstream equipment.
Ammonia is a process gas that is very important in the semiconductor industry for the formation of nitride layers in electronic transistors through chemical vapor deposition and epitaxy processes. More specifically, ammonia is commonly used for the formation of silicon nitride and silicon oxynitride films by direct nitridation of silicon oxide. Growing films of silicon nitride and silicon oxynitride requires ammonia of very high purity. Ammonia is also used in the production of compound semiconductors such as GaN, GaAlN, etc. Oxygen is a particularly harmful contaminant because its high chemical reactivity leads to its incorporation as an impurity into films during thermal nitridation of silicon oxide or during the production of GaN and GaAlN semiconductors. The manufacturing of GaN films requires very large amounts of ammonia during the processing steps. New methods of ammonia purification are thus required to reduce the cost of manufacturing wafers in the GaN process. Regenerable purifiers are also becoming more important as the cost of ownership becomes a more critical parameter.
Water vapor and carbon dioxide gas are two other detrimental contaminants in hydride gases, and which can lead to the degradation of products formed by deposition of active layers of metals or metal compounds from a hydride gas environment. Water is one of the most common and yet most difficult impurities to remove from process gases. Most drying processes can reduce the moisture content of a gas to only a xe2x80x9cminimumxe2x80x9d which is still in the parts per million (ppm) range. However, in the manufacture of components such as high performance light emitting diodes (LEDs), the water content of the depositing gases must be reduced to the parts-per-billion (ppb) range.
Many of the purifier materials such as reduced nickel or copper catalysts that are effective in removing contaminants in more inert gases, such as nitrogen and the noble gases, are not effective in purifying hydride gases. The reactivity of the hydride gases such as ammonia quickly causes degradation of the purifier materials and contaminates the gas stream.
Prior art processes have been described in which oxygen has been removed from ammonia streams by various purifiers. U.S. Pat. No. 5,716,588 to Vergani, et al., describes removing oxygen from ammonia and other hydride gases at room temperature using purifier comprising elemental iron and manganese deposited on an inert material.
U.S. Pat. No. 6,241,955 to Alvarez, Jr. et al, describes high surface metal oxides (greater than 100 m2/g) for removing contaminants such as oxygen, carbon dioxide, and water vapor from hydride gases. The metal oxides are not deposited on a substrate, but rather are used as bulk material within a gas-purifier canister. Since only the active sites on or near the outer surface of the bulk material are accessible to and capable of removing impurities from the gas stream, the Alvarez, Jr., et al., purifier needs to have a larger surface area to be effective.
There remains a need in the art for a reagent that removes contaminants such as oxygen and water from hydride gases. Further, there remains a need for a reagent that is more efficient (removes more contaminants per square meter of surface area) than the prior art. Moreover, there is a need for a purifier material that can be regenerated. Further, a need exists for purifier materials that remove oxygen from hydride gases without concurrently emitting contaminants such as moisture into the purified gas stream and without causing substantial decomposition of the hydride gas.
Accordingly, this invention provides a purifier material capable of reducing the level of contaminants such as oxygen and moisture in a hydride gas stream to parts-per-billion levels or sub-parts-per-billion levels. The purifier materials of this invention comprise a thin layer of reduced forms of an oxide of a metal deposited or coated onto the surface of a nonreactive substrate. The reduced forms of the metal oxide thin layer coating include one or more reduced oxides of the metal in which the oxidation state of the metal is lower than the maximum oxidation state of the metal. In addition to the reduced oxides of the metal, the thin layer may further include the completely reduced form of the metal (i.e., the metal in a zero oxidation state). In one embodiment, the total surface area of the thin layer coating is less than 100 m2/g.
This invention further provides methods of producing a purifier material of this invention, comprising:
(a) providing a precursor comprising a nonreactive substrate having deposited thereon a thin layer of a metal of a first oxidation state;
(b) heating the precursor under a flow of nitrogen at a temperature between about 100xc2x0 C. and 600xc2x0 C. for a period of time; and
(c) treating the precursor from step (b) under reductive conditions capable of reducing the oxidation state of the metal, thereby producing a purifier material comprising the substrate having deposited thereon a thin layer of one or more reduced forms of an oxide of the metal having a second oxidation state, wherein the second oxidation state is lower than the first oxidation state.
This invention further provides methods of purifying hydride gases comprising contacting the contaminated hydride gas with a purifier material of this invention.
Additional novel features and advantages of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods particularly pointed out in the appended claims.
This invention provides methods of producing purifier materials for removing contaminants such as oxygen and moisture from hydride gases. The purifier materials produced by the methods of this invention are capable of reducing the level of contaminants such as oxygen and moisture in a hydride gas stream to parts-per-billion levels or sub-parts-per-billion levels.
In one embodiment, a purifier material of this invention is produced by the method comprising:
(a) providing a precursor comprising a nonreactive substrate having deposited thereon a thin layer of a metal of a first oxidation state;
(b) heating the precursor under a flow of nitrogen at a temperature between about 100xc2x0 C. and 600xc2x0 C. for a period of time; and
(c) treating the precursor from step (b) under reductive conditions sufficient to reduce the oxidation state of the metal in the precursor thin layer, thereby producing a purifier material comprising a nonreactive substrate having deposited thereon a thin layer of one or more reduced forms of an oxide of the metal of a second oxidation state, wherein the second oxidation state is lower than the first oxidation state.
In another embodiment, a purifier material of this invention may be produced from a precursor comprising a substrate having deposited thereon a thin layer of a first form of the metal, wherein the purifier material is produced by changing the form of the metal in the thin layer without changing the oxidation state of the metal. For example, in one embodiment the precursor comprises a nonreactive substrate coated with a salt form of the metal. The treatment step comprises treating the precursor with nitrogen, followed by treating the precursor under conditions sufficient to change the form of the metal (e.g., by decomposition of the metal salt) without changing the oxidation state. This embodiment thus produces a purifier comprising a substrate coated with a thin layer of a metal oxide, wherein the oxidation state of the metal oxide of the purifier is the same as the oxidation state of the metal salt of the precursor. Further, the oxidation state of the metal in the purifier thin layer is lower than the maximum oxidation state possible for that metal.
As used herein, the term xe2x80x9cprecursorxe2x80x9d refers to a nonreactive substrate coated with a thin layer of a metal having a first oxidation state or a thin layer of a first (non-oxide) form of a metal. The oxidation state of the metal of the precursor thin layer need not be in its maximum oxidation state.
In one embodiment, the thin layer of either the precursor or the purifier material comprises a monolayer thickness of the particular form of the metal coated or deposited onto the surface of the nonreactive substrate. However, the thin layer may also comprise two or more layers of the particular form of the metal coated onto the surface of the nonreactive substrate. The terms xe2x80x9cthin layer,xe2x80x9d xe2x80x9ccoating,xe2x80x9d and xe2x80x9cthin layer coatingxe2x80x9d are used interchangeably herein.
As used herein, the term xe2x80x9cnonreactive substratexe2x80x9d refers to a material that is stable under both oxidative and reductive conditions, i.e., a material that is neither oxidized nor reduced under reaction conditions such as those used in the methods of this invention. Examples of nonreactive substrates suitable for purposes of this invention include, but are not limited to, alumina, amorphous silica-alumina, silica (SiO2), aluminosilicate molecular sieves, titania (TiO2) and zirconia (ZrO2). The substrates are commercially available in a variety of shapes of different sizes, including, but not limited to, beads, sheets, extrudates, powders, tablets, etc.
The surface of the precursor substrate can be coated with a thin layer of a particular form of the metal (e.g., a metal oxide or a metal salt) using methods known to those skilled in the art, including, but not limited to, incipient wetness impregnation techniques, ion exchange methods, vapor deposition, spraying of reagent solutions, co-precipitation, physical mixing, etc. In addition, many such coated precursors are commercially available.
As used herein, the terms xe2x80x9cmetal having a first oxidation statexe2x80x9d and xe2x80x9cfirst form of a metalxe2x80x9d refer to the form of the metal comprising the thin layer coated onto the surface of the precursor. For example, in one embodiment the precursor coating comprises a thin layer of a metal having a first oxidation state which is consequently treated to produce a purifier material comprising a nonreactive substrate coated with a thin layer of one or more oxides of the metal having a second, lower oxidation state. In another embodiment, the precursor coating comprises a first form of the metal, wherein the first form is other than a metal oxide. In this embodiment, the precursor is treated to produce a purifier material comprising a nonreactive substrate coated with a thin layer of a metal oxide having the same oxidation state as the first form of the metal. Examples of a xe2x80x9cmetal having a first oxidation statexe2x80x9d and xe2x80x9cfirst form of a metalxe2x80x9d include, but are not limited to, an oxide, a salt, an acid, an organic complex or an inorganic complex of the metal. Examples of metals suitable for purposes of this invention include, but are not limited to, vanadium, molybdenum, antimony, bismuth, tin, cerium, chromium, cobalt, copper, tungsten, and mixtures thereof. Suitable metal salts for purposes of this invention include, but are not limited to, nitrates, carbonates, oxalates, etc.
To produce the purifier materials of this invention, the coated precursor is preferably first heated under a flow of nitrogen to remove most of the moisture contained in the precursor. It was observed that the removal of water prior to the treatment step (e.g., the reduction step or the decomposition step) allowed the subsequent steps to proceed more efficiently, as discussed below. In one embodiment, the precursor is heated under a flow of nitrogen at a temperature between about 100xc2x0 C. and 600xc2x0 C. for a period of time sufficient to remove the majority of the moisture from the precursor. In one embodiment, the precursor is heated under nitrogen for a period of time between about 1 and 200 hours. The precursor is contained in a sealed canister or reactor during the nitrogen treatment, and is maintained in this sealed environment after the nitrogen treatment to prevent recontamination of the precursor with moisture prior to the reduction step.
In one embodiment, the nitrogen-treated precursor is then treated under reductive conditions sufficient to reduce the oxidation state of the metal in the thin layer from a first oxidation state to a second oxidation state, wherein the second oxidation state is lower than the first oxidation state. The product obtained after the reduction step is a purifier material having deposited thereon a thin layer of one or more reduced forms of a metal oxide, wherein the oxidation state of each metal oxide form is lower than the oxidation state of the metal in the precursor coating. However, the reductive conditions used in the methods of this invention do not completely reduce all of the metal in the precursor coating to the elemental metal.
In one embodiment, the reduction step comprises contacting the nitrogen-treated precursor with 100% hydrogen gas at an elevated temperature for a time sufficient to reduce the precursor thin layer metal of a first oxidation state, thereby forming a coating of one or more oxides of the metal having a second, lower oxidation state. In one embodiment, the precursor is heated under a flow of 100% hydrogen at a temperature between about 300xc2x0 C. and 600xc2x0 C. for a period of time between about 1 and 200 hours. The resulting purifier material comprises a nonreactive substrate coated with a thin layer comprising one or more reduced forms of an oxide of the metal. In one embodiment, a percentage of the thin layer of the purifier material further contains the metal in its zero oxidation state.
In another embodiment, the reduction step comprises contacting the nitrogen-treated precursor with a gaseous mixture comprising hydrogen and an inert or nonreactive gas such as argon, helium, or nitrogen, where the mixture comprises from about 0.1% up to 100% hydrogen. Reaction variables such as time, temperature, etc., are selected such that the reductive conditions are sufficient to reduce the precursor thin layer metal of a first oxidation state, thereby forming a coating of one or more oxides of the metal having a second, lower oxidation state.
In another embodiment, the reductive conditions comprise contacting the nitrogen-treated precursor with a gaseous mixture comprising hydrogen and a reactive gas such as ammonia, where the mixture comprises from about 0.1% up to 100% hydrogen. Reaction variables such as time, temperature, etc. are selected such that the reductive conditions are sufficient to reduce the precursor thin layer metal of a first oxidation state, thereby forming a coating of one or more oxides of the metal having a second, lower oxidation state.
In another embodiment, the reduction step comprises contacting the nitrogen-treated precursor with either 100% ammonia, or with a gaseous mixture comprising ammonia and an inert or nonreactive gas such as argon, helium, or nitrogen, where the mixture comprises from about 0.1% up to 100% ammonia. Reaction variables such as time, temperature, etc. are selected such that the reductive conditions are sufficient to reduce the precursor thin layer metal of a first oxidation state, thereby forming a coating of one or more oxides of the metal having a second, lower oxidation state.
In another embodiment, the oxidation state of the metal in the precursor thin layer is reduced by reacting the nitrogen-treated precursor with a reducing agent such as hydrazine, hydrazine derivative, lithium aluminum hydride, etc., either in a liquid solution or in the gas phase, according to methods known in the art. Reaction variables such as time, temperature, etc., are selected such that the reductive conditions are sufficient to reduce the precursor thin layer metal of a first oxidation state, thereby forming a coating of one or more oxides of the metal having a second, lower oxidation state.
In one embodiment, a purifier material may be produced from a precursor comprising a nonreactive substrate coated with a thin layer of a first form of the metal, such as a metal salt, wherein the treatment step comprises decomposing the metal salt to produce a thin layer of a metal oxide without changing the oxidation state of the metal. In this embodiment, the oxidation state of the metal in the precursor coating is lower than the maximum oxidation state possible for that metal. In one example, the precursor comprises a thin layer of cerium (III) nitrate coated onto the surface of an alumina substrate. In this embodiment, the precursor is treated with nitrogen, followed by subjecting the precursor to conditions that will decompose the cerium (III) nitrate to cerium (III) oxide.
In general, the final purifier material comprises about 1 to 20% of the reduced forms of the metal oxide and about 80 to 99% of the substrate. In one embodiment, the total surface area of the thin layer of the final purifier material is less than 100 m2/g. In another embodiment, the total surface area of the thin layer of the final purifier material is between about 10 and 95 m2/g.
As used herein, the terms xe2x80x9creduced forms of an oxide of the metalxe2x80x9d and xe2x80x9cmetal oxide having a second, lower oxidation statexe2x80x9d refer to one or more oxide forms of the metal in which the metal has a lower oxidation state than that of the metal in the precursor thin layer. The thin layer of a final purifier product of this invention may contain one or more different metal oxides. Thus, the term xe2x80x9csecond oxidation statexe2x80x9d is not limited to one specific oxidation state, but rather encompasses different oxide forms of the metal, wherein each of the metal oxides in the purifier coating has an oxidation state that is lower that that of the metal of the precursor coating.
For example, in one embodiment the metal oxide thin layer of a precursor is a molybdenum oxide. Molybdenum is known to form at least four oxides, which are, in descending order of oxidation state of molybdenum, MoO3, Mo2O5, MoO2, and Mo2O3. Thus, if the precursor comprises a thin layer of MoO3 (in which the oxidation state of Mo is +6), then the reduction step can produce a final purifier material having a thin layer that contains one or more of the lower oxides of molybdenum, including Mo2O5, MoO2, and Mo2O3. In addition to the one or more reduced forms of molybdenum oxide, a percentage of the thin layer of the purifier material may also contain metallic molybdenum (Mo), i.e., molybdenum in its zero oxidation state. The composition of the thin layer of the purifier material will of course depend on the amount of time the precursor is exposed to hydrogen gas during the reduction step, as well as the temperature during the reduction. Alternatively, if the precursor comprises a thin layer of Mo2O5 (in which the oxidation state of Mo is+5), then the thin layer of the final purifier material may comprise one or more of the lower oxides of molybdenum, including MoO2 and Mo2O3, and may further contain molybdenum in its zero oxidation state (Mo).
It is not necessary that the first oxidation state of the metal in the precursor thin layer be the maximum oxidation state for that metal. However, at least a portion of the metal in the final purifier thin layer is a reduced metal oxide. That is, the second oxidation state of the metal in the final product thin layer is between the first oxidation state of the metal of the precursor layer and the zero oxidation state of the metal. While not wishing to be bound by any theory, it is believed that the partially reduced metal oxide forms of the metal produce a more stable coating on the nonreactive substrate, as opposed to a fully reduced metal coating, in which the elemental metal could sinter before or during a gas purification process, thereby losing most of its efficiency.
The present invention also includes methods of purifying contaminated hydride gases using the purifier materials of this invention. More specifically, this invention provides a method of removing contaminants such as oxygen and moisture from a hydride gas stream comprising contacting the contaminated gas stream with a purifier material for a period of time sufficient to reduce the level of the contaminants to parts-per-billion levels. The purifier material of this invention is generally contained within a gas-purifier chamber having a gas inlet and outlet. Many container configurations are well known to those skilled in the art. In one embodiment the container is made of a material that is resistant to hydride gases, such as stainless steel. Alternatively the container can be coated on the inside surface with a hydride-resistant material. The purifier materials of this invention can be made in situ (i.e., within the container to be used in a gas purification system). Alternatively, the purifier materials can be made in a separate container and subsequently transferred to a gas-purifier container.
The purifier materials can be used in any compatible gas purifier system in the art, including systems used to purify hydride gases for use in semiconductor processes. The method of purifying hydride gases generally comprises flowing the contaminated hydride gas through a container in which a purifier material of this invention is retained. Typically, hydride gas flow rates during a purification process are in the range of about 1 to 200 standard liters per minute (splm). Operating temperatures of the hydride gases can range from about 0xc2x0 C. to 70xc2x0 C.
As used herein, the term xe2x80x9chydride gasxe2x80x9d includes, but is not limited to, ammonia (NH3), arsine (AsH3), phosphine (PH3), germane (GeH4), silane (SiH4), methyl silane, t-butyl arsine, t-butylphosphine, and alkyl or halide derivatives thereof, and any gaseous compound containing an active hydride moiety, or mixtures thereof.
One of the advantages of a purifier material of this invention is that it comprises a thin layer coating of one or more reduced forms of the metal oxide on the surface of the substrate. As a result, all or substantially all of the active sites of the thin layer are exposed to the contaminated hydride gas during the purification step, thus increasing the efficiency of the thin layer to trap and thus remove contaminants from the hydride gas stream. This is a significant improvement over bulk metal oxide purifiers in the art which are not coated onto substrates, since many of the active sites in the bulk purifiers are not situated at or near the surface of the bulk material and therefore are not accessible to the contaminants.
A further advantage of the purifier materials of this invention is that they can be regenerated more efficiently, as discussed below in Examples 2 and 3, thus extending the lifetime of the purifier material. In addition, the purifier materials of this invention are potentially more stable than other purifier materials in the art.
The purifier materials of this invention are suitable for purifying gases for semiconductor and other electronic substrate manufacturing, including the hydride gases as described above, as well as other reactive gases including, but not limited to, trimethyl aluminum, trimethyl gallium, and trimethyl indium. More generally, the purifier materials of this invention are useful for removing contaminants from any gas that is used for deposition of component materials for any other type of high purity product where moisture and/or oxygen content is detrimental to the product""s production. This could include, for instance, production of high purity substrates or other types of materials for research purposes, production of optical fibers and optoelectronic devices (including lasers) and production of high purity materials that are to be used in extreme environments such as products for use in spacecraft or artificial satellites.
The performances of several purifier materials provided by this invention are illustrated in Examples 2, 3, 4 and 5 below.